Fix sys_prctl.pr_cap_ambient.

This test didn't clear errno before testing it, so was sensitive to
whatever code had run before. Since I'd already added the code to dump
the exact kernel version before realizing the actual bug, I've left that
in too.

Bug: http://b/121156651
Test: ran tests
Change-Id: I38482036cb6f035c0c98c27eb64d17829f29b09a
diff --git a/tests/sys_prctl_test.cpp b/tests/sys_prctl_test.cpp
index 7afa626..cd23c0a 100644
--- a/tests/sys_prctl_test.cpp
+++ b/tests/sys_prctl_test.cpp
@@ -20,6 +20,7 @@
 #include <sys/capability.h>
 #include <sys/mman.h>
 #include <sys/prctl.h>
+#include <sys/utsname.h>
 #include <unistd.h>
 
 #include <string>
@@ -75,10 +76,14 @@
       "https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/"
       "?id=b7f76ea2ef6739ee484a165ffbac98deb855d3d3";
 
+  utsname u = {};
+  ASSERT_EQ(0, uname(&u));
+
+  errno = 0;
   auto err = prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_CLEAR_ALL, 0, 0, 0);
   EXPECT_EQ(0, err);
   // EINVAL -> unrecognized prctl option
-  ASSERT_NE(EINVAL, errno) << "kernel missing required commits:\n"
+  ASSERT_NE(EINVAL, errno) << "kernel (" << u.release << ") missing required commits:\n"
                            << caps_sha << "\n"
                            << caps_typo_sha << "\n";